In order to increase transmission capacity in an optical communication system, a polarization multiplexing system employing QPSK (Quadrature Phase Shift Keying) modulation system is disclosed in non patent literature 1, for example. In non patent literature 1, a polarization beam splitter separates a polarization multiplexed optical signal into two optical signals whose polarization are orthogonal each other. Next, a 90-degree hybrid mixes each separated optical signal with local oscillation light and outputs optical signals corresponding to an in-phase component and a quadrature component, respectively. And then, a photodiode converts each of the output optical signals of the 90-degree hybrid into an electric signal.
In non patent literature 1, since polarization demultiplexing is realized by digital signal processing based on this electric signal, an adjustment is not performed to match a polarization plane of the polarization multiplexed optical signal with a polarization plane of the polarization separated by the polarization beam splitter which is a reference axis of the polarization beam splitter. That is to say, the electric signal output by each photodiode includes signal components transmitted by each polarization wave respectively. However, filter coefficients are obtained which represent the relation to each signal component, and each signal component is restored from each electric signal by digital processing. The fact is utilized for calculating the filter coefficient in non patent literature 1 that the amplitude of the QPSK signal of the modulation system employed is constant. In this way, the polarization demultiplexing in the polarization multiplexed QPSK optical signal by the digital signal processing disclosed in non patent literature 1 does not need the adjustment of optical polarization plane, and a highly-reliable system is realized at low cost.    Non Patent Literature 1: Seb J. Savory et al., “Electronic compensation of chromatic dispersion using a digital coherent receiving device”, OPTICS EXPRESS 2120, Vol. 15, No. 5, March, 2007.